Multicast routing is gaining acceptance as a technology for distributing content. With multicast routing a multicast tree is used to connect a source with receivers and data is sent across the multicast tree towards receivers. Data is not copied at the source, but rather, inside the network at distribution branch points. Only a single copy of data is sent over links that lead to multiple receivers, resulting in bandwidth saving. Multicast packets are replicated in the network at the point where paths diverge by routers enabled with Protocol Independent Multicast (PIM), and other supporting multicast protocols. Unlike broadcast, the traffic is only received and processed by devices that are listening for it.
Thus, multicast routing is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to potentially thousands of subscribers.
Two multicast service models are deployed today:                (a) Any Source Multicast (ASM) where an interested receiver of a multicast session notifies the network via Internet Group Management Protocol (IGMP) that it is interested in joining a specific group associated with that multicast session. The receiver then receives content sent by any source sending to this group. This model is targeted to support dynamic multi-source sessions for different applications, e.g. for conferencing and financial trading. The standard protocol set in support of ASM is IGMPv2 or IGMPv3 for hosts to join a group and Protocol Independent Multicast-Sparse Mode (PIM-SM), together with Multicast Source Discovery Protocol (MSDP), for inter-domain operations and rendezvous point (RP) redundancy; and        (b) Source Specific Multicast (SSM) where an interested receiver of a multicast session specifies both the group and the source (or sources) from which it would like to receive content. The SSM model is achieved through the use of IGMPv3 which allows the host to specify both the group and the sources of interest, as well as the PIM-SSM which generates S, G joins.        